Worldwide hundreds of millions of tons of Municipal Solid Waste (MSW) are produced every year. One of the most common techniques to deal with such amounts of waste is incineration, as this reduces waste mass and volume while providing recovery of energy from the waste to generate electricity. Although the process of Municipal Solid Waste Incineration (MSWI) is a convenient means for reducing the mass and volume of MSW, this process does lead to still significant amounts of ash as the by-product that is produced during the combustion of municipal solid waste in combustor facilities. The ashes from MSWI may be disposed as landfill. However, in most developed countries, where land is scarce and environmental controls are tight, recycling and reusing of waste ashes is preferred over landfill disposals.
An important aspect of the recycling and reuse of MSWI ashes is that the ash material may vary, particularly because the composition of municipal solid waste varies over time and from country to country, due to differences in regulations, lifestyle and waste treatment processes. The properties of MSWI ashes can be divided into two groups: physical properties, such as particle size distribution, moisture content, bulk density, compressive strength, permeability and porosity, and chemical properties, such as chemical composition, loss on ignition, heavy metals and leachability, organic constituents and chloride content. Generally, the chemical and physical composition of ash will depend on the compositions of the raw MSW feed, the operational conditions, the type of incinerator and air pollution control system design.
In most modern mass burn solid waste combustors, several individual ash streams are produced. They include grate ash, siftings, boiler ash, scrubber ash and precipitator or baghouse ash. The term bottom ash is commonly used to refer to the grate ash, siftings and, in some cases, the boiler ash stream. Approximately 90 percent of the bottom ash stream consists of grate ash, which is the ash fraction that remains on the stoker or grate at the completion of the combustion cycle. It contains small amounts of unburnt organic material and chunks of metal. The grate ash stream consists primarily of glass, ceramics, ferrous and nonferrous metals, and mineralic slags. It comprises approximately 75 to 80 percent of the total combined ash stream.
The term fly ash is used to refer to the ash collected in the air pollution control system, which includes the scrubber ash and precipitator or baghouse ash. The baghouse or precipitator ash comprises approximately 10 to 15 percent of the total combined ash stream. This material includes many so-called fines, i.e., particles with a diameter of less than 1 mm, e.g., 100 micrometer. In Europe, most combustor facilities separate the bottom ash and fly ash streams. As the bottom ash constitutes the largest volume, approximately 75 to 80 percent, of the total amount of ash material after MSWI, methods and apparatus have been developed to recycle and reuse these ashes.
Typically, the ash material after MSWI is moist, and includes a mix of metal, glass and mineral particles of various sizes. The MSWI ash includes many fines, as described above. The presence of moisture and fines causes water bonds (H bonds) between particles of different material, so that smaller agglomerated particles are formed. Due to these agglomerations, it is difficult to separate and retrieve the particles of different materials from the moist MSWI ash without washing or drying steps, which are costly, time consuming and environmentally less friendly.